A polyurethane foam molded article having a high density surface layer (hereinafter referred to as a “skin”) has very good surface property in applications other than those where it is covered by a skin made of other material such as mattress and cushion. Therefore, a flexible polyurethane foam and a semi-rigid polyurethane foam are typically used in furniture, automobile interior components such as an arm rest, a steering wheel and a shift lever knob, shoe soles, and sporting goods. A rigid polyurethane foam has many applications such as an imitation lumber and a structural material.
Articles such as automobile steering wheel and shoe sole which require high wear resistance, for example, are usually made of a molded member covered by a skin on the surface thereof (or on the bottom in the case of shoe), with the inner portion (core) desirably having a lower density for better touch feeling.
Saddles of bicycles are also commonly covered by a skin because a high strength is required of the surface with which the rider makes contact and the portion where the saddle is mounted on the bicycle, with the core desirably having a lower density and softness for better ride comfort. In order to finish the painted surface with better appearance, the surface layer is required to have a high density.
In the case of a structural material made of a rigid polyurethane foam, too, it is known to provide a skin in order to finish the painted surface of the molded article with better appearance, while it is required to make the core from a foamed material of low density in order to reduce the weight of the molded article.
The polyurethane foam molded article will have higher commercial value and find wider applications if a skin and/or a high density portion is formed at a portion where it is required during use and the core is made of a foamed material having lower density and lower hardness in accordance to the required performance.
As a blowing agent used in foaming of a polyurethane resin, water and a so-called low boiling point liquid such as fluorinated hydrocarbon have been used. However, it is very difficult to form a skin on the polyurethane foam molded article by using water which is a commonly used blowing agent. Therefore, the so-called low boiling point liquid such as fluorinated hydrocarbon has been used in the production of the polyurethane foam molded article having a skin.
There are known molding technologies and apparatuses for producing the polyurethane foam molded article, where three or more kinds of polyurethane forming components containing the blowing agent based on the low boiling point liquid are weighed and mixed, with the apparatus designed so that materials containing the blowing agent based on the low boiling point liquid are weighed in constant proportions, mixed and discharged continuously. As a result, proportions of the skin and the foamed core of the molded article remain substantially constant and the skin can be formed over the entire surface of the molded article. However, it is not possible to form the skin selectively on those portions of the molded article which actually require the skin, and to change the hardness of those portions and change the density of the molded article itself.
Carbon dioxide gas, instead of fluorinated hydrocarbon, can be used as the blowing agent, and carbon dioxide foaming technology has recently been developed in which a molded foam article is produced from a raw liquid of polyurethane with carbon dioxide gas added beforehand. However, the addition of carbon dioxide to the polyol component tends to hinder the reaction and does not ensure the production of satisfactory polyurethane foam molded articles. In the case where carbon dioxide is added to isocyanate, because the amount of carbon dioxide dissolved in isocyanate is kept constant, it is not possible to form the skin selectively on portions of the molded article that actually require the skin, and to change the hardness of those portions and change the density of the molded article itself.
Thus, it has been conceived to supply carbon dioxide gas as a third component from a metering pump directly to a mixing head of a blowing machine, and an apparatus including the blowing machine has been developed. A molding technology which employs such an apparatus, however, is also intended for producing a molded article of uniform density. As such, it is designed to supply the raw materials containing carbon dioxide gas in constant mix proportions from the start to the end of charging the raw liquid of polyurethane into the mixing head. As a result, it is not possible to form the skin selectively on portions of the molded article which actually require the skin, to form a low density foamed core and to change the proportions of the skin and the low density core and the hardness of those portions or to change the density of the molded article itself.
A method for producing a polyurethane foam molded article by using a blowing machine of polyurethane resin is described in Polyurethane World Congress '97, P-185, wherein components required for foaming, such as an isocyanate, a polyol, a catalyst, a crosslinking agent and a low boiling point liquid such as fluorinated hydrocarbon, are charged separately by metering pumps from respective tanks into the mixing head.
JP-A-11-293027 discloses a method for producing a low density polyurethane foam by charging a polyol which is saturated with carbon dioxide dissolved therein as a blowing agent, a polyisocyanate component and a polyol component separately into a mixing head.
JP-A-2003-334828 discloses a method for producing a low density polyurethane foam with improved efficiency of entrapping carbon oxide by charging carbon dioxide into a raw liquid of polyurethane comprising a polyisocyanate component and a polyol component in a mixing head.
In the method described in Polyurethane World Congress '97, P-185, in order to obtain a molded article having homogeneous density and hardness, the blowing machine is configured to discharge the components required for blowing in predetermined quantities and continuously discharge a raw material of fixed composition. As a result, although a stable polyurethane foam molded article having substantially constant proportions of the skin and the foamed core is obtained, it is not possible to produce, in a single stage, a polyurethane foam molded article having the skin and high density portions selectively on portions of the molded article which actually require the skin and high density portions by changing the proportions of the skin and the foamed core.
In the method described in Japanese Unexamined Pat. Publication (Kokai) No. 11-293027, although density (foaming ratio) of the polyurethane foam molded article can be stabilized because a polyol saturated with carbon dioxide dissolved therein is used as a blowing agent, it is not possible to produce, in a single stage, a polyurethane foam molded article having the skin or high density portions selectively on portions of the molded article which actually require them by changing the proportions of the skin and the foamed core.
In the method described in JP-A-2003-334828, although a stable and low density polyurethane foam can be formed as a constant amount of carbon dioxide is continuously charged since the blowing agent into the raw liquid of polyurethane, it is not possible to produce, in a single stage, a polyurethane foam molded article having the skin and high density portions selectively on portions of the molded article which actually require the skin by changing the proportions of the skin and the foamed core.
Thus, although the conventional methods which employ the blowing agent are capable of molding the polyurethane foam molded article having uniform performance in a single stage, there are no methods which can produce, in a single stage, the molded article having the skin and high density portions selectively on portions of the molded article actually requiring them, which has the changed density or hardness of each portion by changing the proportions of the skin and the high density portions and the foamed core.